Cathode ray tube having a non-discoloring x-ray absorptive display window



n- 1969 J. DE GIER 3,422,298

CATHODE-RAY TUBE HAVING A NON'DISCOLORING X'RAY ABSORPTIVE DISPLAYWINDOW Filed March 15. 1966 INVENTOR.

JOHAN NES DE GIER United States Patent 6504105 U.S. Cl. 31364 7 ClaimsInt. Cl. H01j 29/ 00, 31/00 ABSTRACT OF THE DISCLOSURE A cathode-raytube having a display Window on the inner side of which a luminescentscreen is disposed which luminesces under the impact of an electronbeam. The window comprises of a first portion adjacent the screen ofpoor electrical conductivity and containing a relatively smallpercentage of readily reducible compounds so that it will not discolorunder impact of the electron beam while a second portion remote from theluminescent screen consists of a glass composition which contains leadoxide and cerium oxide in amounts eifective to absorb X-rays generatedin the tube and transmitted through the first portion withoutdiscoloring due to the X-rays being absorbed on the electron impact.

The invention relates to a cathode-ray tube provided with a luminescentscreen disposed on a glass window, which window has a composition suchthat it has a poor electrical conductivity and/ or that it contains alow percentage of readily reducible compounds such as oxides of lead,antimony and arsenic.

It is known that the glass window of a cathode-ray tube operating at ahigh voltage, for example, above 15 kv. may exhibit a graduallyincreasing discolouration due to a prolonged electron bombardment. Suchvoltages are used, for example, in cathode-ray tubes for projectiontelevision, for radar, for flying spot scanning, for colour televisionand in large cathode-ray tubes (having a diameter of, for example, morethan 42 cm.) for black-andwhite television. This discolouration may beaccounted for by the reduction of oxides present in the glass due tocharge transport. This discolouration is therefore counteracted byensuring that the glass has a poor electrical conductivity or that itcontains a low percentage of readily reducible compounds, whilealternatively a combination of both steps may be used. Since the sodiumoxide con tent is substantially exclusively determinative of theconductivity of a kind of glass, if a very poor electrical conductivityis required, the glass should not contain more than 1% by weight ofsodium oxide. The content of readily reducible compounds does not exceedapproximately 7.5% by weight in this case. In practice, it is lesssimple to prepare glass having a low content of sodium oxide, since sucha kind of glass is excessively hard and its expansion coeflicient isexcessively low 'while it is not readily meltable. It is thereforerather ensured that the glass contains a very low percentage of readilyreducible compounds, i.e., not higher than 2.5% by weight, moreparticularly not higher than 1% by weight, in which 3,422,298 PatentedJan. 14, 1969 case the sodium oxide content does not exceed 10% byweight. The permissible quantity of readily reducible compounds dependsupon the sodium oxide content, while these two magnitudes in turn dependupon the actual charge of the cathode-ray tube during operation. Thesaid readily reducible compounds do not all contribute to the sameextent to the discolouration of the glass. For example, a given contentof lead oxide brings about a much stronger discolouration than the samequantity of antimony oxide, so that in a given case the permissiblequantity of antimony oxide is greater than that of lead oxide.

In cathode-ray tubes operating at a high voltage of, for example, above15 kv., due to the electron bombardment of the luminescent substance andthe glass, also X- rays are produced which also bring about adiscolouration especially in lead-containing glass, while at a very highvoltage such a discolouration also occurs in the absence of lead if theglass contains, for example, antimony, arsenic, barium or aluminium, asthe case may be in the form of impurities. As is known, thisdiscolouration may be counteracted by adding cerium oxide to the glass.Consequently, more particularly the glass window then contains ceriumoxide.

These X-rays the intensity of which strongly depends upon the actualcharge of the cathode-ray tube, may penetrate through the glass window.In order to protect the environment, it should consequently be preventedthat X-rays, if they surpass a given intensity, can reach theenvironment. X-rays are readily absorbed by glass containing oxides ofheavy metals, more particularly leadcontainin'g glass, but as set outhereinbefore, the glass window should preferably contain a very lowpercentage of lead oxide in order to prevent this window from beingdiscoloured due to the electron bombardment. In practice, it has beenfound to be difficult to avoid the discolouration of the glass atvoltages upwards of 25 kv. and nevertheless to ensure that the X-raysproduced are absorbed to a sufficient extent, inter alia also in thecase in which the glass window contains 7.5 by weight of lead oxide,since at a high charge the window of a small tube does not have asuflicient thickness to absorb the X-rays to a sufficient extent. Theinvention provides a solution in which on the one hand thediscolouration of the glass window due to the electron bombardment iscounteracted, while on the other hand the environment is protectedagainst X-rays.

According to the invention, the cathode-ray tube is provided on theouter side at the area of the glass window with a pane containing leadoxide and cerium oxide. The composition of the glass window prevents itfrom being discoloured due to the electron bombardment, while the X-raysproduced in the cathode-ray tube are absorbed on the one hand in thepane as a result of the presence of the lead oxide, while on the otherhand they do not bring about a discolouration in the pane owing to thepresence of the cerium oxide. The thickness of the glass pane amounts,for example, to 3 to 6 ms. The intensity of the X-rays depends upon theactual charge of the cathoderay tube. In general, a pane containing from5 to 15% by weight of lead oxide and up to 1% by weight of cerium oxidewill prove satisfactory.

Cathode-ray tubes provided with a pane are known per se. In given cases,a pane is used for protection 3 4 against implosion. 'Ihis pane thenhas, for example, the under IV and V a few compositions of the pane 6.These following composition: compositions are stated in percentages byweight.

Percent by weight TABLE S10 68.1 Lizo 049 r I II III IV V Na O a K20 RbO BaO PbO A1203 ---i L3 Sb O 0.52 Remainder 0.34 9- 2 0. Furthermore,especially in cathode-ray tubes for repro- Q34 ducing colour televisionimages, a pane is used which is generally frosted in order to reduce thereflection of the What is claimed is: light in the environment. Thispane has, for example, the A Cathode'ray tube havlhg a WlhdOW dlsplayfoll wi i i of an image produced by impact of an electron beam on apercent b i h luminescent screen disposed on the lnternal surface ofsaid s o 75 window, said window comprising a first portion adjacent 0172 to the luminescent screen and consisting of a glass como ()2position containing a relatively low percentage by weight A1203 ofreadily reducible compounds and having a relatively c o 7 g poorelectrical conductivity whereby said portion does not o 3 discolor underimpact thereon of the electron beam, and Remainder Q3 a second portionremote from said screen consisting of a glass composition containinglead oxide and cerium oxide The Procfiss of manufacturing the P y Startfrom in amounts which are effective to absorb X-rays generated a fiatglass Plate which is allowed to bulge during heating in said tube andpassing through said first portion without so that the concave shapecorresponds with that of the b i di l d h b or h l t o b Window of theCathode-fay tube, whereupon the P is 2. A cathode-ray tube as claimed inclaim 1, in which adhered t0 the windowthe first window portion does notcontain more than 10% In another method, the pane is moulded into thedeby weight f Sodium id sired shape, whereupon it is adhered to thewindow of the 3 A cathode-ray b as l i d i l i 2, in hich tube. thefirst window portion does not contain more than 1% The lnvention furtherrelates to an apparatus comb i h of di id prising a cathode-ray tubeprovided with a luminescent 4 A cathode-ray t b as l i d i l i 2, i whih Screen disposed on a glass Window, which Window has a the first windowportion does not contain more than 2.5% composition such that it has apoor electrical conducb i h of dil d ibl id tivity and/or that itcontains a low percentage of readily 5 A h d b as l i d i l i 3 i hi hreducible compounds such as oxides of lead, antimony thg fi t Windowportion does not contain more than and arsenic. According to theinvention, in the apparatus by Weight f readily reducible i on the frontslde of the cathode-ray tube a pane 15 P 6. A cathode-ray tube asclaimed in claim 1, in which vided which contains lead oxlde and ceriumoxide. The the first Window pOrtion contains cerium Oxide composition ofthe glass window prevents it from being 7. A cathode ray tube as claimedin claim 1, in which discoloured due to the electron bombardment, whilethe the Second Window Portion contains f 5 to 15% by X-rays produced inthe cathode-ray tube are absorbed on Wdght of lead oxide and up to 1% byWeight of cerium the one hand in the pane provided in the apparatus dueto oxide the presence of the lead oxide, while on the other handReferences Cited they do not bring about a discolouration in the panedue to the presence of the cerium oxide. Also in this case, a paneUNITED STATES PATENTS containing from 5 to 15% by weight of lead oxideand 2,293,529 8/1942 Bedford 313-64 X up to 1% by weight of cerium oxidewill generally prove 2,579,709 12/ 1951 Smith et a1. 10653 satisfactory.2,676,109 4/ 1954 Barnes et a1 313-64 X Apparatus provided with a paneon the front side of 2,747,105 5/1954 Fitzgerald et a1. 106-53 thecathode-ray tube are known per se. Such a pane 2,856,303 10/1958Armistead 106--53 serves, for example, for protection against implosion.3,209,191 9/1965 Hamilton 3131l0 The invention will now be describedmore fully with 3,356,579 12/1967 Harrington 106-53 reference to adrawing the figure of which is a partially diagrammatic sectional viewof a cathode-ray tube. The 00 FOREIGN PATENTS envelope of the tubeconsists of the window 1, the cone 2 8587667 5/1940 France and the neck3. Inside the tube a luminescent screen 4 2 Provided 11 the Windovg ighile 3 3 8;" th ii JOHN W. HUCKERT, Primary Examiner.

lagrammatica y is arrange in e nec or e sa e of clarity, the remainingparts of the interior of the tube 65 JAMES Assmam Exammer' are notshown. The pane 6 is adhered to the window 1 US Cl XR of the tube. Thetable indicates, by way of example,

under I, II and III a few compositions of the window and 106-63; 2202.1;313-317

